System and method using portable wall engaging ferromagnetic particle impregnated target medium for electromagnetically measuring distance between oposing walls of a structure

ABSTRACT

Both a system and a method are provided which allows an eddy current probe to accurately determine both the proximity and dimensions of non-conductive structures which are normally invisible to such probes. The system comprises a portable target medium that is movable into a known position with respect to the non-conductive structure, and that includes a conductive material that couples strongly with a fluctuating magnetic field, and a movable eddy current probe that emanates a fluctuating magnetic field and which generates a signal indicative of the magnitude of the interaction between the field and the portable target medium from which the distance between the two may be computed. The system may be used to determine the proximity of non-conductive structures such as plastic pipes that have been buried under ground, as well as the dimensions of such non-conductive structures. When the system is applied to measure the dimensions of such a structure, the portable target medium assumes a form that is flexibly conformable to one of the walls of the structure and which is placed in abutting relationship thereto. The eddy current probe is then scanned against an opposing wall of the structure, whereby the width of the structure may be computed by measuring the strength of the interaction between the probe and the medium. In addition to measuring the dimensions of non-conductive structures, the system may be used to measure the dimensions of non-magnetic structures when the target medium includes a strongly magnetic material.

This is a Divisional application of Ser. No. 07/662,664, filed Feb. 28,1991 now U.S. Pat. No. 5,200,704.

BACKGROUND OF THE INVENTION

This invention generally relates to a system and method that enables aneddy current probe to determine the proximity of non-conductivestructures, and the dimensions of both non-conductive and non-magneticstructures by the use of a portable target medium.

Eddy current probes for inspecting the condition of structures formedfrom conductive materials are well known in the prior art. Such probesgenerally comprise a sensor coil, a multi-frequency generator forconducting high frequency alternating current through the coil in orderto generate a fluctuating electromagnetic field, and a circuit formeasuring the amount of impedance experienced by an alternating currentas it flows through the coil windings. When the sensor coil of such aneddy current probe is placed in the vicinity of a structure formed froma conductive material, the electromagnetic field emanated from the coilcouples with the conductive material and induces a counter-flowingalternating magnetic field in the material. This counter-flowingmagnetic field in turn induces eddy currents in the material. Thecounter-flowing electromagnetic field and eddy currents imposes animpedance on the electromagnetic field emanated by the sensor coil whichmay be precisely measured by the impedance measuring circuit of theprobe. In some applications, the measured impedance applied to thesensor coil of the probe is used merely to detect the presence orabsence of a particular conductive structure, such as the presence ofmetallic objects buried in the sands of a beach. In other applications,the measured impedance is used to detect the presence or absence ofstructural faults such as cracks, pits, or areas of wall thinning in ametal pipe or other conductive structure. In all applications, theimpedance experienced by the fluctuating electromagnetic field emanatedby the sensor coil measurably changes as the coil is scanned around thevicinity of a conductive structure, both as a function of the distancefrom the sensor coil to the structure, and as a function of physicalvariations in the structure, such as changes in wall thickness, thepresence or absence of current-impeding cracks or other faults in thestructure, or changes in the conductivity of the material used to formthe structure. Because of the need in such areas as the nuclearengineering arts to be able to accurately and remotely inspect thestructural integrity of reactor and steam generator components inhostile environments, a large body of sophisticated knowledge andexpertise has been developed that is aimed at extracting detailedinformation about a metallic structure being scanned by an eddy currentprobe.

Unfortunately, eddy current probes cannot be used to detect or inspectstructures formed from non-conductive materials, since the fluctuatingelectromagnetic field emanated by the coil sensor cannot couple withnon-conductive materials. While there are alternate modes of inspectionsuch as ultrasonic probes which do not require the structure beingexamined to be formed of electrically conductive materials, thesealternate inspection modes are sometimes difficult if not impossible toimplement. For example, the use of ultrasonic inspection probes requiresthe presence of a liquid couplant, such as water, between the probe headand the structure being inspected. In certain applications, such as theinspection of remote components of electrodynamic machinery, it may behighly undesirable, if not impossible, to provide such a liquid couplantaround the structure. Moreover, even in instances where the nature ofthe structure or its accessibility or its environment does not pose amajor obstruction to the application of a liquid couplant around thestructure, there are some non-conductive structural materials that areinherently uninspectable by ultrasonic probes, such as porous ceramics.Any such liquid couplant would penetrate and be retained by the porousnature of such ceramics . Thus, there is no known satisfactory techniquefor inspecting the walls or measuring the wall thicknesses of the smalldiameter, thin walled porous ceramic tubing used for fuel cell and gasfiltering applications.

Additionally, even in the case of structures which are conductive butnot formed of non-magnetic materials, there are instances where neithereddy current nor ultrasonic inspections are capable of accuratelydetermining the dimensions of such structures. For example, in assessingthe wall thickness variations in the Zircaloy® guide tubes used innuclear fuel assemblies, it is possible for an eddy current probe toyield inaccurate results due to differences in conductivity along thelength of the tube caused by differences in the orientation of thezirconium crystals. Additionally, an ultrasonic probe is notsatisfactorily accurate across the ten foot length of such tubes becausethe axial taper present within the small diameter of such tubes (whichhave an outer diameter of only about 0.50 inches) prevents sufficientcontrol of the interrogating sound beam.

Clearly, both a system and method are needed for both detecting thepresence and for measuring the dimensions of remotely-located structuresformed from non-conductive materials which is as accurate and reliableas the state-of-the art eddy current probe inspections made ofstructures formed from conductive materials. Ideally, a system andmethod would allow such inspections to be performed easily, cheaply, andremotely and would make maximum use of commercially available inspectionequipment. Finally, it would be desirable if such a system and methodwere capable of accurately measuring the dimensions of any structuremade from any non-conductive or non-magnetic material, and in particularstructures formed from non-conductive or non-magnetic materials where anultrasonic or eddy current probe is either incapable of achievingaccurate results, or highly undesirable or impossible to apply.

SUMMARY OF THE INVENTION

The invention is both a system and method for determining both theproximity and dimensions of a non-conductive structure thatadvantageously utilizes eddy current probe technology. The systemcomprises a portable target medium that is movable into a known positionwith respect to the structure, wherein the medium includes a conductivematerial that couples with a fluctuating magnetic field, and a movableeddy current probe for emanating such a fluctuating magnetic field andfor generating a signal indicative of the distance between the targetmedium and the probe so that the position of the structure relative tothe probe can be determined.

The system and method are particularly useful in informing a systemoperator as to the position of a non-conductive structure, such as aplastic pipe, that has been buried in the ground. In such anapplication, the portable target medium is preferably in the form of aflexible sheet material that has been buried along with the pipe in aposition adjacent to one of the pipe walls. In the preferred embodiment,the sheet material is a strip of polyethylene that has been impregnatedwith a substance which strongly couples with a fluctuating magneticfield, such as particles of a ferromagnetic material. To enhance themagnitude of the coupling between the ferromagnetic particles in thefilm and the fluctuating electromagnetic field emanated by the eddycurrent probe, the polyethylene film is shaped into rectangular stripswhich are horizontally oriented above their respective pipes in aparallel relationship. These strips of polyethylene film preferablyinclude a plurality of drain apertures so that the film does not impededrainage to the ground, and is further preferably color coded toindicate one or more of the characteristics of the pipe, i.e., pipediameter, pipe material, or the type of liquid being conducted by thepipe.

Alternatively, the system of the invention may be used to determine oneor more of the dimensions of such a non-conductive structure, as forexample the diameter of a piece of plastic or ceramic tubing. In thisparticular application of the invention, the portable target medium ispreferably conformable to one of the walls of the structure. Inoperation, the eddy current probe is positioned against an opposing wallof the structure and actuated. The magnitude of the electromagneticinteraction between the probe and the target medium is then measured,which in turn may be used to compute the distance between the probe andthe target medium.

The target medium may be a flexible strip of sheet material which isflexibly conformable to the contours to one of the walls of thestructure whose dimensions are being measured. In this embodiment of thesystem, the target medium may have adhesive on one side for temporarilysecuring the medium into conforming contact with one of the walls of thestructure. Alternatively, the target medium may be a bladder filled witha liquid that strongly interacts with the fluctuating electromagneticfield emanated by the eddy current probe, such as ferrofluid (which isan aqueous solution of colloidally-suspended ferromagnetic particles),or mercury. The target medium may also assume the form of a paint whichwhen dripped on a non-conductive structure leaves a film on one of itsside walls that strongly interacts with the fluctuating field emanatedby an eddy current probe.

In the method of the invention, the magnitude of the interaction betweenthe eddy current probe and the target medium is first measured for aplurality of different distances in order to calibrate the probe. Next,the target medium is placed into a known spatial relationship withrespect to the non-conductive structure. If the method is being usedmerely to detect the proximity of underground, non-conductive pipes, thetarget medium may assume a strip-like form as previously described thatis buried a known distance above the pipes in parallel relationship. Ifthe method is being used to measure one or more dimensions of anon-conductive structure, then the target medium may be secured inabutting relationship to one of the walls of the structure.

In the next step of the method, the eddy current probe is scanned aroundthe vicinity of the structure and points of maximum interaction betweenthe electromagnetic field and the target medium are taken note of. Wherethe method used is to locate underground pipes, the position of theprobe upon such maximum interaction should be directly above the pipebeing located. Where the method is being used to measure a dimension ofa structure, such maximum interaction occurs when the probe is abuttedagainst an opposite wall of the structure, and the distance between theprobe and the target medium is at a minimum. The magnitude of theelectromagnetic interaction between the eddy current probe and thetarget medium is then measured, whereupon the distance between the probeand the target medium is determined. In the case where the method isbeing used to determine the proximity of buried pipe, this last stepinforms the operator as to how deep the pipe is being buried.Alternatively, where the method is being used to measure a dimension ofa non-conductive structure, this last step precisely informs the systemoperator of the distance between two opposing walls of the structure.

Both the system and method of the invention advantageously enablessophisticated and accurate eddy current probes to be used to detect thepresence of non-conductive structures which are normally "invisible" tosuch probes. Additionally, both the system and the method of theinvention may be used to accurately measure either non-conductivestructures, or non-magnetic structures whose material properties make itimpossible to make such measurements with standard eddy current orultrasonic probes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates how the system and method of the invention may beused to determine the proximity of a plastic pipe by means of an eddycurrent probe assembly, and a strip-like target medium buried directlyabove the plastic pipe;

FIG. 2 is a graph illustrating how the response of the eddy currentprobe assembly increases as the probe coil of the assembly is placedcloser into a position directly over the strip-like target mediumillustrated in FIG. 1;

FIG. 3 is a perspective view of a target medium which may be used toimplement the embodiment of the system and method of the inventionillustrated in FIG. 1;

FIG. 4 is an enlarged view of the section of the strip illustrated inFIG. 3 surrounded by dotted lines, illustrating both the drain aperturesand the particulate ferromagnetic material that are present in thisstrip-like target medium;

FIG. 5 schematically illustrates how the system and method of theinvention may utilize the combination of an eddy current probe assembly,and a target medium that is conformable to the contours of one of thewalls of a non-conductive structure in order to measure the thickness ofthis structure;

FIG. 6 is a graph illustrating how the magnitude of the response of theeddy current probe assembly changes as a function of the distancebetween the probe coil of the assembly and the conformable target mediumapplied over one of the wall of the structure;

FIG. 7A is a perspective view of one of the embodiments of theconformable target medium of the system of the invention which is formedfrom a flexible sheet material impregnated with ferromagnetic particlesin combination with an adhesive layer for detachably securing the targetmedium over one of the walls of the structure being measured;

FIG. 7B illustrates an alternate embodiment of the conformable targetmedium used in the system of the invention which is formed from a flat,thin-walled bladder that has been filled with a liquid which stronglyinteracts with the fluctuating electromagnetic field emanated by theeddy current probe assembly;

FIG. 8 is a schematic, perspective view illustrating an embodiment ofthe system of the invention that is well adapted for monitoring thethickness of a sheet material that is formed between the nip of a pairof opposing rollers;

FIG. 9 illustrates how the system and method of the invention may beused to measure the varying thickness of a non-magnetic metallic tubewhose inner walls are tapered by means of a magnetic liquid targetmedium, and

FIG. 10 illustrates the output of the eddy current probe assembly usedin the embodiment of the system illustrated in FIG. 9, showing how themagnitude of the response of the eddy current probe assembly varies withthe thickness of the walls of the tube.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 illustrate how both the system and the method may be usedto determine the proximity of a non-metallic structure such as thepolyvinyl chloride pipes that are used more and more frequently totransport water, gas and oil. During the initial installation of suchplastic pipes 1, a trench 3 of a desired depth is first dug in theground 5. Next, coarse gravel 7 is placed at the bottom of the trench toprovide a support bed for the pipe 1 that is capable of draining waterto the surrounding ground 5. After the pipe 1 has been laid, it is nextsurrounded by a layer of medium to fine gravel 9 again for the purposeof facilitating drainage. In the prior art, the layer of medium to finegravel 9 was covered with ground filler 10, and vegetation 17 wasallowed to grow all around the area of the trench 3 which made itdifficult, if not impossible to precisely locate the plastic pipe 1 inthe event that a repair had to be made. However, in the instantinvention, a target medium 11 in the form of a polyethylene strip 13impregnated with ferromagnetic particles is horizontally positioned overthe layer of medium and fine gravel 9 prior to the final burial to thepipe 1 under the ground filler 10. This target medium 11 forms one ofthe two major components of the system of the invention. The other majorcomponent of the system is an eddy current probe assembly 20 having atleast one probe coil 22 which is electrically connected to amultifrequency generator 24. In the preferred embodiment, the probe coil22 is actually a plurality of concentrically arranged coils, each ofwhich is independently connected to the multifrequency generator 24. Asis specifically described in U.S. Pat. No. 4,855,677 by William G.Clark, Jr. et al., and assigned to the Westinghouse ElectricCorporation, the provision of a plurality of concentrically arranged,and independently connected eddy current coils allows the systemoperator to extract more detailed information from the readings obtainedfrom the multifrequency generator 20, and the text of this patent ishereby expressly incorporated herein by reference. The multifrequencygenerator 24 includes circuits for both applying a multitude of highfrequency, alternating voltages to the coils contained within the probecoil 22 of the system, as well as an impedance measuring circuit formeasuring the extent to which the lines 26 of flux emanated by the probecoil 22 interact with the target medium. In the preferred embodiment,the multifrequency generator 24 is a MIZ-18 multifrequency generatormanufactured by Zetec located is Isaquah, Wash.

With reference now to FIGS. 3 and 4, the polyethylene strip 13 whichforms the target medium 11 in this particular embodiment of the systemof the invention may be supported on a spool 30 prior to being dispensedinto the planar position illustrated in FIG. 1. A plurality of drainapertures 32 are provided within the polyethylene strip 13 so that watermay freely pass through the strip 13 and on down through the layer ofmedium and fine gravel 9, through the coarse gravel 7, and ultimatelyinto the surrounding ground 5. As is schematically indicated in FIG. 4,this polyethylene strip 13 is impregnated with ferromagnetic particles35 which interact strongly with the fluctuating magnetic flux 26emanated by the probe coil 22 of the eddy current probe assembly 20. Torender the polyethylene strip 13 easily detectable by the eddy currentprobe assembly 20 without significantly interfering with the structuralintegrity of the surrounding polyethylene, the strip 13 preferablycomprises between 0.5 and 10 percent of ferromagnetic particles byweight, and more preferably between 1 and 3 percent ferromagneticparticles by weight. While not specifically indicated in any of theseveral figures, the polyethylene strip 13 is preferably also colorcoded with one or more bright stripes of bars of color to indicatevarious structural characteristics of the pipe (i.e. , diameter, wallcontents, depth of entrenchment, and fluid material conducted ) .

The first embodiment of the method of the invention may be best beunderstood with reference to FIGS. 1, 2 and 3. In the first step of thismethod, a target medium 11 in the form of the previously describedpolyethylene strip 13 is unreeled from the spool 30 prior to the finalburial of the plastic pipe 1 underneath the ground filler 10 and laid inthe horizontal position illustrated in FIG. 1 directly above the pipe 1and all along its length. Next, after vegetation 17 has grown up allalong the surrounding area so that it is difficult if not impossible tolocate the borders of the trench 3 which holds the pipe 1, the probecoil 22 of an eddy current probe assembly 20 is actuated and scannedaround the vicinity of the plastic pipe 1. During the scanning step, thesystem operator is careful to note the points at which the response ofthe impedance-sensing circuit within the multifrequency generator 24registers its maximum output. At the registration of such an output, theprobe coil 22 of the eddy current probe assembly 20 should be directlyover the strip-like target medium 11, and hence directly over theplastic pipe 1 itself. In the final steps of this method, the systemoperator continues to scanningly move the probe coil 22 of the eddycurrent probe assembly 20 in order to determine the orientation of thepipe. Of course, pipe 1 will be oriented along the line L thatcorresponds to the maximum response of the eddy current probe assembly20 (see FIG. 2). The location of this line L is marked by fluorescentroad cones, or surveyor's markers or the like in order to provide arecord of the location of the underground target medium 11, and hencethe underground plastic pipe 1.

FIGS. 5 and 6 illustrate how the system and method of the invention maybe used to measure the dimensions of a non-conductive structure 38. Inthis embodiment of the system of the invention, the target medium 40 isformed from a material which is conformable to at least one of the walls42 of the non-conductive structure 38. In implementing this particularembodiment of the method of the invention, the probe coil 22 of an eddycurrent probe assembly 20 as previously described is abutted against andscanned over an opposing wall 44 of the non-conductive structure.However, the output of the multi frequency generator 24 is conducted toa microprocessor 45 as shown. The conductive eddy current probe assembly20 and microprocessor 45 used in this embodiment of the system ispreferably a Model KD-4000 measuring displacement system manufactured byKaman Instrumentation (Industrial Products Division) located in ColoradoSprings, Colo.

The method of the invention implemented by this particular embodiment ofthe system of the invention may be best understood with reference toboth FIGS. 5 and 6. In the first step of this method, the eddy currentprobe assembly 20 is calibrated by moving it in the vicinity of a targetmedium 11 which is electromagnetically identical to the specific,conformable target medium 40 that has been applied to one of the walls42 of the non-conductive structure 38. The specific magnitude of theresponse of the eddy current probe assembly 20 at specific distancesbetween the probe coil 22 and sample target medium are recorded andentered into the memory of the microprocessor 45. Next, the eddy currentprobe assembly 20 is actuated, and the probe coil 22 is scanned over anopposing wall 44 of the non-conductive structure 38 as is schematicallyillustrated in FIG. 5. The interaction between the fluctuating lines 26of magnetic flux, and the conformable target medium 40 is recorded foreach particular spatial axis (of which only the x axis is shown in FIG.6 for simplicity). The magnitude of this response for each point alongeach of the spatial axes on the opposing wall 44 is recorded by themicroprocessor 45. Finally, the microprocessor 45 compares the magnitudeof these responses with the magnitude of the responses entered into itsmemory in the initial calibration step, and generates a graph such asthat illustrated in FIG. 6 which not only informs the operator as to theshape of the non-conductive structure 38 along a particular axis, butalso the absolute thickness of the structure 38 at all points along thisaxis. Of course, the absolute thickness of the structure 38 at allpoints over the area of the opposing wall 44 may be determined by makingmultiple, side-by-side scans over the wall 44 with the probe coil 22.

FIG. 7A and 7B illustrate two of the numerous forms that the conformabletarget medium 40 used in this particular embodiment of the system andmethod of the invention may take. In FIG. 7A, the conformable targetmedium 40 is formed from a flexible sheet material 46 that has beenimpregnated with particles of a ferromagnetic material in the same rangeof concentrations as was previously discussed with respect to thepolyethylene strip 13 illustrated in FIGS. 3 and 4. Additionally, thisflexible sheet material 46 includes a layer 48 of adhesive on one of itssides to allow this particular embodiment of the conformable targetmedium 40 to be detachably secured onto one of the walls 42 of anon-conductive structure 38 in much the same fashion that vinylelectrical tape might be applied around the surface of a plastic pipe orother structure. FIG. 7B illustrates another embodiment of theconformable target medium 40 that-is formed from a thin-walled bladder50 of a plastic material (such as polyvinyl chloride) that has beenfilled with a liquid target medium 52. The liquid target medium 52 maybe a colloidal suspension of ferromagnetic particles in a liquid such aswater or kerosene (known as ferrofluid in the chemical arts) or a liquidmetal as mercury. While the bladder-embodiment of the conformable targetmedium 40 illustrated in FIG. 7B requires an external means of supportin order to affix it into position against a wall of a non-conductivestructure, it is well suited for the dimensional measuring of delicatestructures which could not withstand the tensile forces involved inpulling off the tape-like embodiment of the conformable target medium 40illustrated in FIG. 7A.

Further embodiments of the system and method of the invention areillustrated in FIGS. 8 and 9, wherein the conformable target medium 40assumes the form of a target roller 58, and a freely flowing liquid 52.The embodiment of the system illustrated in Figure 8 is particularlywell adapted for measuring variations in the thickness of a sheetmaterial 54 that is extruded between the nip of a pair of opposingrollers 56a,b. The previously-mentioned target roller 58 intimatelycontacts one side of the finished sheet material 54, while a battery 62of eddy current probe coils 22 wipingly engages the other side of theextruded sheet material 54 directly opposite from the target roller 58.The exterior of the target roller 58 is covered with a resilient foam,rubber or elastomer which has been impregnated with the previouslymentioned ferromagnetic particles in approximately the sameconcentration as the previously discussed target medium 11 formed from apolyethylene strip 13. The resilient nature of the target material 60that forms the exterior surface of the roller 58 allows it to maintainintimate contact with the underside of the extruded sheet material 54despite any local variations in the thickness or surface texture of thissheet material 54. Additionally, the use of a battery 62 of eddy currentprobe coils 22 allows the eddy current probe assembly 20 to accuratelymonitor variations in the thickness of the extruded sheet material 54 atall points across its width.

FIGS. 9 and 10 illustrate how both the system and method of theinvention may be used to measure the wall thickness of a structureformed from a conductive, but non-magnetic material such as theZircaloy® tubing 64 used in nuclear fuel assemblies. Such tubing 64includes opposing, tapered walls 66 in its interior. Due to the ten footlengths of such tubes 64, and their relative small internal diameters(which may be as little as 0.25 inches) it is impossible to accuratelymeasure the varying wall thickness by ultrasonic probes, since thenarrow inner diameter does not permit sufficient control of theorientation of the interrogating sound beam. Moreover, even though thewalls 66 of such tubing 64 are electrically conductive, it is difficultif not impossible to utilize eddy current probes to accurately measurethe thickness of these walls due to the fact that the conductivity ofthe wall 66 may change along the ten foot length of the tube 64 as aresult of differences in the orientation of the zirconium crystals inthese walls. By contrast, the thickness of the tapered walls 66 iseasily measured by both the system and method of the invention by firstfilling the interior diameter of the tube 64 with a liquid target medium52 which contains fine particles of a ferromagnetic material thatinteracts strongly with the fluctuating electromagnetic field emanatedby the probe coil 22 of the eddy current probe assembly 20. In themethod of the invention implemented by this particular system, the eddycurrent probe assembly 20 is first calibrated by measuring the specificmagnitude of the eddy current response for specific distances betweenthe probe coil 22, and a volume of liquid target medium 52 havingprecisely the same physical and electromagnetic properties as the liquidtarget medium 52 which fills the inner diameter of the tube 64. Next,the probe coil 22 is scanned along the longitudinal axis of the tube asis schematically illustrated in FIG. 9. As the thickness of the opposingwall 66 increases, the eddy current response diminishes as isgraphically illustrated in FIG. 10. In the final steps of this method,the magnitude of the measured eddy current responses are converted intomeasured wall thicknesses by the microprocessor 45, and these wallthicknesses are associated with specific coordinates along thelongitudinal axis of the tube 64. In this manner, the method of theinvention is capable of determining whether such Zircaloy® tubes 64 arefit for use in such important applications as nuclear fuel assemblies.Of course, the invention may be similarly applied to measure thedimensions of any structure made from a conductive but non-magneticmaterial.

We claim:
 1. A system for measuring the distance between two opposingwails of a structure formed from a non-magnetic material, comprising:1)a portable, conformable, and integrally formed target medium meanshaving a surface for conformingly engaging a surface of a wall of saidstructure to follow contours in said wall and which includes adhesivemeans for engaging said surface of said target medium means to saidsurface of said wall, and particles of a ferromagnetic materialimpregnated in a solid, non-conductive material that strongly interactwith a fluctuating electromagnetic field, wherein each portion of saidtarget medium means interacts with said field a known magnitude for agiven distance, and 2) an eddy current probe means movable into abutmentagainst an opposing wall of said structure for generating a fluctuatingelectromagnetic field and detecting the magnitude of the interactionbetween said field and said target medium means so that the distancebetween said opposing walls of said structure can be determined.
 2. Asystem for measuring the distance between two opposing walls of astructure formed from a non-magnetic material, comprising:1) a portableand conformable target medium means having a surface for abuttinglyconforming to a wall of said structure to follow contours in said walland which includes means for affixing substantially every point on thesurface of said target medium means to a point on a surface of a wall ofsaid structure, and particles of a ferromagnetic material impregnated ina solid, non-conductive material that strongly interact with afluctuating electromagnetic field, wherein each portion of said targetmedium means interacts with said field a known magnitude for a givendistance, and 2) an eddy current probe means movable into abutmentagainst an opposing wall of said structure for generating a fluctuatingelectromagnetic field and detecting said target medium means so that thedistance between said opposing walls of said structure can bedetermined.
 3. A system as defined in claim 2, wherein said targetmedium means is a section of sheet material sufficiently flexible tofollow and abut against the contours of a wall of said structure andwherein said affixing means includes an adhesive on one side fordetachably securing said material along the contours of said wallstructure.
 4. A method for measuring the distance between two opposingwalls of a structure formed from a non-magnetic material by means of aportable, conformable and integrally formed target medium means thatincludes particles of a ferromagnetic material impregnated in a solid,non-conductive material that strongly interact with a fluctuatingelectromagnetic field; wherein each portion of said target medium meansinteracts with said field a known magnitude for a given distance, and aneddy current probe means movable into abutment against a wall of saidstructure that generates a fluctuating electromagnetic field, comprisingthe steps of:adhering a surface of said portable, conformable targetmedium means against a surface of a wall of said structure such thatsaid target medium means follows contours in said wall; engaging saideddy current probe means on an opposing wall of said structure such thatsaid fluctuating electromagnetic field generated by said probe meansinteracts with said particles of ferromagnetic material in said targetmedium means; monitoring the strength of the interaction between thefluctuating electromagnetic field of said probe means and said targetmedium means to determine the distance between said first and secondwalls, and removing said target medium means from said wall.
 5. A methodas defined in claim 4, wherein said target medium means is detachablyadhered to said first wall, and further comprising the step of removingsaid target medium means from said first wall after said distancebetween said first and second walls has been determined.
 6. A method asdefined in claim 4, wherein said eddy current probe is slidably scannedover said second wall, and wherein the strength of said interaction ismonitored as a function of the position of said scanning eddy currentprobe to determine the shape of said contours of said first wall.